This application claims priority from Japanese Application 2000-391313 filed Dec. 22, 2000 and Japanese Application 2000-391324 filed Dec. 22, 2000, the entireties of which are incorporated herein by reference.
The present invention relates to a driving module for a laser diode used in optical communication devices and the like, and particularly relates to a driving module equipped with a parallel laser array diode having a plurality of laser diodes. More particularly, it relates to an improvement in the power line connection between a driving driver IC and a laser diode.
As network transmission capacity represented by the Internet or mobile traffic increases, a demand for optical link throughput enhancement grows. In an optical transmission system, especially the type used in short-range situations, an optical parallel module having a laser diode array of a parallel multi-channel structure has recently been used.
A structure as shown in the block diagram of FIG. 6 is conventionally known as a transmission module used in such a case. This structure has a driving module for laser diodes of a parallel multichannel structure constructed by a submount substrate 35 on which a plurality of laser diodes 34, 34, are arranged side by side along with a driver IC 36.
A ceramic substrate is used in the submount substrate 35 to satisfy the heat radiation and electrical connection demands of the plurality of mounted laser diodes 34. An electric current is supplied to each laser diode 34 by a power pattern 37 juxtaposed with the line of laser diodes 34. Electric current supplying pads 37a, 37a are formed on left-hand and right-hand substrates at both ends of this power pattern 37. An electric current path is normally formed by wire bonding from a power line of a package on which the submount substrate 35 is mounted.
The driver IC 36 has an output buffer circuit 39 and electrical current switches 38, 38 . . . . The electrical current switch circuits function to turn on/off the electric current flowing through the laser diodes 34 in accordance with an input signal. Further, electric current supplying pads 40a, 40b, 40c, . . . are arranged for every circuit, and then the electric current is supplied to the driver IC 36 by connecting these pads to the power line of the package by wire bonding.
Thus, in the conventional power electric current supplying system used to supply both the laser diode and the electric current switch circuit, the electric current path flowing from the power line into the electric current switch circuit is usually different when the laser is turned on and off.
To improve a transmission data rate of an optical link module, the laser diode and the driver IC are important elements. However, when the transmission rate is high, a fluctuation of the laser driving electric current is often generated in the conventional construction. Such fluctuation causes time delay and phase delay of the laser diode current, and therefore a distortion of the waveform of an optical signal occurs. In particular, in the case of an array laser having a plurality of laser diodes arranged side by side, a change in the electric current (when turning on and off of the electric current) supplied to each laser diode is influenced by an adjacent channel operation, and causes a so-called electrical cross talk between adjacent laser diodes.
Concretely, in the case of an electric current supply as shown in FIG. 6, one example of the waveform of the laser driving current at a high operating speed is provided as shown in FIG. 7. Further, one example of the waveform of an applied voltage of the power pattern 37 of the submount substrate 35 is provided as shown by a characteristic curve B of FIG. 8. FIG. 7 shows one laser diode current waveform in a specific portion when adjacent other laser diodes are simultaneously operated. A curve P shows a case in which other portions are operated in a reverse phase (the other laser diodes are turned off), and a curve Q shows a case in which other laser diodes are operated in the same phase (the other laser diodes are also turned on). These curves show changes in the phase of the electric current waveform when the plural laser diode lines are simultaneously operated. The characteristic curve B of FIG. 8 shows a change in voltage (Vcc) of the power pattern 37 on the submount substrate 35 at a circuit operating time.
Thus, as a result of observations of the laser driving electric current waveform and the fluctuation of the voltage at the power pattern on the submount substrate, a rising time change and the phase delay of the laser diode current n were very large, depending upon a driving state (the same phase or the reverse phase) of the other adjacent laser channels. Accordingly, the signal waveform of the laser emitting light by this driving electric current is also seriously distorted, and this distortion has become an obstructing factor in the improvement of the transmission data rate. Further, when a time change of this driving electric current is caused, the applied voltage of the power pattern 37 on the submount substrate 35 is also greatly changed. This change causes the interaction between the channels. The time change of the laser electric current becomes notable in an NRZ (Not Return Zero) signal having a transmission speed of e.g., 2 Gbps or more, and a high speed signal line transmission of seven stages or more in PN, and has become a large factor of a transmission bit error occurrence.
In view of the above problems, an object of the present invention is to provide a driving module of a laser diode for reducing the time dependent change (waveform distortion and phase delay) of a driving electric current generated in the high speed operation of a laser diode, and reducing the interaction between respective channels and stably performing high speed signal transmission when multichannels are particularly set by mounting plural laser diodes.
With respect to the time dependent change (the waveform distortion and the phase delay) of the laser driving electric current, the inventors have ascertained a large cause of this time change. That is to say, from the results of the experiments, the time dependent change occurs because the electric current paths from a power source to a laser diode and to an electric current switch of a driver IC are different from each other.
Namely, the electric current switch of the driver IC plays a switching role of the electric current path with the laser diode as a load, and the electric current path in which a resistor arranged within the driver IC is set to a load. However, it has become clear that the time dependent change of the laser driving current supplied from the power source in accordance with various kinds of transmission signal lines is generated when the frequency of the electric current flowing through the electric current path is increased as the transmission rate is increased. Additionally, it has become clear that the time dependent change is caused by a difference between an electric impedance through a transmission line (e.g., a conductive pattern on the submount substrate and a connecting wire) to the laser diode, and an electric impedance through a transmission line to the load resistor arranged within the driver IC.
In a multichannel module arranging a plurality of laser diodes therein, the electric current paths to the arranged laser diodes individually have different electric impedances, and partially have a common path (e.g., the power pattern 37 formed on the submount substrate 35 described in FIG. 6). Therefore, it has become clear that these factors cause an increase in the cross talk (interaction) between the channels in which the laser driving electric current of a certain channel is influenced by another channel electric current having the common path.
To solve the above problems, one embodiment of the invention is characterized in that a driving module of a laser diode comprises a laser diode substrate mounting a plurality of laser diodes thereto; and a driver circuit substrate having electric current switch circuits for turning on/off the laser diodes; wherein a power line is juxtaposed with one of said laser diodes and one of said electric current switch circuits, and an electric current is supplied from the power line to both the laser diode and the electric current switch circuit.
In a preferred embodiment, a set of the laser diode and the electric current switch circuit for operating the laser diode receives the supply of electric current from approximately the same point on the power line. Preferably, a plurality of sets of laser diodes and electric current switch circuits are arranged, and each set receives the supply of electric current from approximately the same point.
The power line is a conductor pattern juxtaposed with the laser diode on the laser diode mounting substrate. Preferably, the electric current is supplied to the electric current switch circuit by extending the power line toward the electric current switch circuit from a portion close to a laser diode connecting portion of the power line, arranging a connecting pad at a tip of the power line, and making a wire bonding connection with the electric current switch circuit from the connecting pad.
In another embodiment, the electric current switch circuit has an ECL (Emitter Coupled Logic) circuit, and a constant electric current is approximately consumed even in a turning-off state of the laser diode.
In addition, a capacitor can be inserted between a ground point (GND) and approximately the same point of the power line from where the electric current to both the laser diode and the electric current switch circuit is supplied.